Sprinkle chicken with salt and pepper. In a large skillet coated with cooking spray, cook chicken over medium heat until juices run clear, turning chicken at least once. Remove and keep warm. Cook the onion slices in the same skillet until tender.

Spread butter over cut sides of buns. Place buttered side up on an ungreased baking sheet. Broil 3-4 inches from the heat for 1 minute or until golden brown.